This invention relates to a high fidelity speaker system and more particularly to a speaker system of the highest quality, suitable for stereo applications, wherein the undesirable effects produced by sound wave diffraction at the edges of the speaker enclosure are reduced.
Recent improvements in the recording and reproduction of music by electronic means have advanced the art to the point where the speaker system has become the weakest link in the reproduction chain. One of the principal problems encountered by audio engineers tasked with the design of "hi-fi" speakers is that associated with or arising from the diffraction of sound waves at the edges of most speaker enclosures.
A speaker driver element having a diaphragm or cone with a diameter that is small with respect to the smallest wavelength, will radiate a sound wave into space that is essentially bipolar in nature. If this same driver element is placed symmetrically in the center of one face of a closed cubical enclosure, the radiation will be spherically divergent and the polar response pattern will be approximately spherical, i.e., omni-directional in all planes, so long as the perimeter of the box remains small with respect to a wavelength at the frequency being radiated. However, a portion of the wave radiated by the driver propagates along the outside surface of the enclosure until it reaches the side edges. Here, the wave encounters a sudden discontinuity and, because of the lack of any further supporting surface, its amplitude must go to zero. This results in the production of a wave phenomena known as diffraction. In the process of being diffracted from the side edges of the enclosure, some fraction of this wave is reflected back toward the driver element, another fraction is reradiated into the surrounding space as a new, omni-directional sound source, while the remaining fraction continues propagating around the corner headed for the near edges of the enclosure where the diffraction process is repeated.
Thus, along the front and side faces of the enclosure, the reflected waves interact with the direct wave to form an interference pattern, commonly referred to as a standing wave pattern. So long as the time delay between the original, direct wave, sound and the diffracted sound sources is small in terms of the fraction of a cycle at the frequency being radiated, little destructive interference occurs and the shape of the radiation pattern remains essentially omni-directional. However, as the width of a face of the enclosure approaches a full wavelength, the reflected portion of the diffracted wave generates a standing wave pattern in the form of two juxtapositioned semicircular lobes, together extending for the width of the enclosure face. This results in a relative gain of two since both standing wave maximums are in phase. When the width of the front face of the enclosure is substantially equal to two full wavelengths, the standing wave pattern takes the form of four juxtapositioned semicircular lobes together extending for the width of the enclosure face, with the two inner lobes having a positive polarity and the two outer lobes having a negative polarity. Since each maximum represents an apparent source of sound radiation, cancellation occurs in front of the speaker, which results in a forward null in the polar pattern response of the speaker system, i.e., a null on axis or at zero degrees using conventional geometrical coordinates. As the size of the enclosure increases until a side is wider than several wavelengths, multiple lobes of the above type will appear in the polar pattern.
In addition to the destructive effect on the polar response pattern which diffraction causes, another serious degradation in quality of reproduction will occur. This pertains to the pulse response of the system. For example, suppose that at a given frequency, the width of the aforementioned speaker enclosure is equal to two wavelengths. At this frequency, "ringing" due to the diffracted wave will reach a distant, on-axis listener at least a full wavelength (one cycle) later than the direct wave. If the signal fed to the driver were a "tone-burst" consisting of perhaps three cycles of the subject frequency, the listener would actually hear four or more full cycles instead of three. More complex wave forms such as those experienced in music would appear to be equally distorted because of ringing due to the late arrival of the diffracted wave. The ear would perceive this form of distortion as a tonal change or imbalance rather than a reflected sound such as that normally occurring from the walls of a room because of the much shorter delay time involved.
Further, the amplitude vs. frequency response of such a speaker, measured on axis, will show "drop-outs" at those frequencies corresponding to the geometrical on-axis nulls in the polar pattern response. The off-axis frequency response will likewise not be smooth, and will show a series of "drop-outs." Depending upon the relative strength between the direct and diffracted waves, these amplitude minimums in frequency response may vary from a few decibels (dB) to as much as 30 dB, or more. In addition to the nulls, amplitude maximums of 3 dB or more also occur periodically with rising frequency.
As speaker technology slowly developed from its beginning over 50 years ago, numerous attempts have been made to arrive at a design which minimized the undesirable effects of diffraction. Indeed, during the late 1930's and early 1940's, several attempts were made to "round" the edges of the enclosure as a means of eliminating the sudden discontinuity represented by the sharp, square edge. This provided a partial solution so long as the radius of curvature at the edges was sufficient to reduce diffraction to a minimum. Unfortunately, the required curvature results in an enclosure which is not fully acceptable to the general public at the present time in terms of cosmetic or aesthetic appeal.
In the 1950's, it was found that some of the effects of diffraction could be minimized by mounting the midrange driver and/or tweeter at an asymmetrical location along the front face of a rectangular enclosure. While a proper choice of driver location will result in a general smoothing of a frequency response curve taken "on-axis", i.e., at a large distance in front of the speaker, the "off-axis" frequency response curve will show the usual undulations of amplitude vs. frequency typical of diffraction and standing-wave problems. Such asymmetrical placement of drivers also results in what is called a skewed lobe, i.e., the polar response pattern radiated by the speaker is not centered on the forward, zero degree, axis. This effect may take several different forms: the lobe may be tilted up or down in the vertical plane, it may be "skewed" to the right or left of center, or the maximum of the lobe may be centered while the shape of the lobe is asymmetrical about the zero degree axis. Advantage has been taken of this characteristic by certain designs which utilize a matched pair, i.e., a "right" position speaker and a "left" position speaker. However, such designs suffer from the fact that the degree of lobe "skew" or "tilt" changes with frequency.
Yet another recent claim for a solution to the diffraction problem is that provided by systems utilizing an "acoustical suspension" type woofer mounted in a closed rectangular box. Four separate midrange and tweeter drivers are mounted in an array above the woofer enclosure, without any enclosures of their own, using small plates and mounting brackets. The polar pattern radiated by some of these drivers is typically that from a dipole (bi-directional), while that radiated by others varies from omni-directional (spherical) to uni-directional. (The highest frequency tweeter, a piezoelectric horn, has a uni-directional lobe about 40.degree. wide between half-power points.) While this technique might initially appear to eliminate diffraction problems, a closer examination will reveal the fact that the wide-angle radiation from the "unbaffled" midrange units results in the generation of reflections between adjacent driver surfaces, between some of the drivers and the top of woofer enclosure, and between some of the drivers and the side panels of the enclosure. The effect of these reflections is the generation of a complex standing-wave pattern that is perhaps at least equal in severity to that experienced with ordinary speaker designs.
Still another solution to the diffraction problem is that afforded by the mounting of speaker drivers along the curved surface of a vertical cylinder. While this works well with smaller drivers such as midrange unit or tweeter, it creates the problem of how to mount the larger woofer. In the most popular version of this type design, the woofer is mounted on the flat bottom side of the cylinder with an annular slot opening between the woofer and the floor. However, this configuration results in a phasing problem between the woofer and the midrange drivers due to the time-lag difference encountered. The use of a very low crossover frequency tends to minimize the fluctuations in frequency response due to the actual time delay between the two drivers, but does not fully solve the resulting degradation in transient response. Also, when relatively wide spacing is utilized between adjacent drivers of a speaker system, undesirable lobing of a type similar to that discussed above will occur.
As a result of the foregoing, it should be apparent that the formation of acoustical standing-waves on the front face and on the sides of a speaker enclosure represent a serious problem confronting the design engineer attempting to formulate the specifications for a hi-fi speaker system. Any design for a speaker enclosure which permits such standing-waves will prove, if accurately measured, to yield relatively poor performance with respect to pulse response and time delay distortion. Also, the standing waves will cause large excursions in amplitude vs. frequency response, as well as the formation of undesirable nulls and lobing in the polar response pattern.